Spindle motor

ABSTRACT

A spindle motor including a bearing means which is not seized under the high speed rotational operation and with reduced fretting wear. Consequently, the densification of the hard disk drive device can be achieved.  
     A spindle motor comprising a spindle shaft, a rotor, and a bearing means interposed between the spindle shaft and the rotor, the bearing means includes inner and outer rings and rolling elements interposed therebetween, wherein the inner and outer rings of the bearing means are made of steel, and the rolling elements are made of ceramic.

BACKGROUND OF THE INVENTION

[0001] 1. Technical Field

[0002] The present invention relates to an improvement of a spindlemotor employed in peripheral devices for computers such as the hard diskdrive device, the video tape recorder (VTR), and so on.

[0003] 2. Description of the Prior Art

[0004] There are a variety of equipments using a spindle motor. Apersonal computer is one of these equipments in which a high speedspindle motor is used in the hard disk drive device of compact sizecomprised in the computer.

[0005] The bearing means used for such spindle motor has a size rangedfrom ID (inner diameter)=4 mm and OD (outer diameter)=8 mm to ID=6 mmand OD=15 mm, such bearing means is related to as a miniature bearing.

[0006] The one of such bearing means has a structure including astraight shaft, an inner ring having an inner raceway formed on itsouter peripheral surface, an outer ring having an outer raceway formedon its inner peripheral surface, and a plurality of rolling elements orballs interposed between the inner raceway and the outer raceway andretained in their position by a retainer.

[0007] The other bearing means of another structure has a spindle shaftof stepped structure in which a reduced diameter portion and a largerdiameter portion are provided. The bearing means including the steppedshaft further comprises an outer ring of cylindrical configurationdisposed around the spindle shaft, and an inner ring having an innerraceway formed on the outer peripheral surface thereof, the inner ringis fit around the reduced diameter portion of the stepped shaft andsecured thereto. A pair of double row outer raceways are formed on aninner peripheral surface of the outer ring. Balls are interposed betweenthe one outer raceway and the inner raceway, and retained in theirposition by a retainer.

[0008] Further, balls are interposed between an inner raceway formedaround an outer peripheral surface of the larger diameter portion of thespindle shaft and the other outer raceway, and retained in theirposition by a retainer.

[0009] Recently, a remarkable development or improvement has beenachieved in the hard disk drive device on miniaturization anddensification. Especially for the hard disk drive device of the sizeequal to or less than 3.5 inch, the densification is increased rapidly.More recently, the hard disk drive device of the size of 2.5 inch to beincorporated into personal computers of the note book type is alsodemanded to have substantially the same memory capacity as that of thehard disk drive device of 3.5 inch in spite of its small size. In orderto enlarge the memory capacity of the hard disk drive device of the sizeof 2.5 inch, it is necessary to increase both of the track recordingdensity and the track density. The presently demanded track density isfrom 10 KTPI to 14 KTPI (TPI: Track Per Inch).

[0010] In either hard disk drive device the size of which is 3.5 inch or2.5 inch, it is necessary to increase the number of revolutions of themagnetic disk or disks to increase the data transfer rate of the harddisk drive device. For example, the hard disk drive device of 3.5 inchrequires the number of revolutions from 5400 rpm to 7200 rpm, and insome cases, the number of revolutions over 10000 rpm is required.

[0011] However, in the bearing means adapted to be incorporated intosuch hard disk drive device of small size, localized stresses aregenerated repeatedly between the balls and the inner and/or outer ringsso that the raceway surfaces and the surfaces of the rolling elementsare tend to be roughened. Thus, the above mentioned bearing meansinvolves disadvantages such as the increasing of the vibration, thedeterioration of the acoustic characteristics (more noises aregenerated), and the reduction of the lifetime of the bearing means.

[0012] In order to prevent the raceway surfaces and the surfaces of theballs from wearing or damaging, the inner and outer rings and therolling elements are normally formed of high carbon chromium steel (SUJ2 or the one similar to SUJ 2), and the hardness of the surfaces areincreased by the hardening process to HRC 58-64. Provided that corrosionresistance is required, martensitic stainless steel (the one similar toSUS 440 C) may be utilized.

[0013] When transporting the spindle motor including a bearing means,the spindle shaft is inhibited in its rotation. When vibrations arecaused to be generated under such circumstances, impact loads areapplied repeatedly between the balls and the raceways of the innerand/or outer rings. In such situation, the balls and the raceways aremetal to metal contact with each other, and no film of grease is formedtherebetween.

[0014] Thus the rolling elements and the raceways are tend to make microslips repeatedly and generate localized wearing i.e. the fretting wear.This fretting wear will affect the vibration and the acousticcharacteristics.

[0015] Although the grease of high viscosity is effective in avoidingsuch fretting wear, the viscous resistance of such grease will increasethe required rotational torque, the demand of the motor, and generatedheat energy. Consequently, the grease of high viscosity is notpreferred.

[0016] As the number of revolutions of the hard disk drive device isincreased as mentioned above, the viscosity of the injected grease tendsto decrease by the heat generated in the bearing means. This willinterfere the formation of the oil film of the grease at the contactregion between the balls and the raceway surfaces, and in the worstcase, there is a risk for seizure caused by the breakage of the oilfilm.

[0017] While the oil film of the grease is not formed on the contactregions between the rolling elements and the raceway surfaces, localizedfrictions are caused to be produced thereon, and the surfaces of theregions are damaged or roughened.

[0018] When the surfaces are roughened, the vibration is increased, theacoustic characteristics are deteriorated, and the lifetime of thebearing means is shortened.

[0019] Among the above mentioned drawbacks, the generation of thevibration is especially undesirable, since the vibration affects therun-out of the spindle motor and impairs the precision of the placementof the magnetic head of the high density hard disk drive device.

[0020] Accordingly, the object of the present invention is to overcomethe above mentioned problems. In accordance with the present invention,a spindle motor including a bearing means which will not seize under thehigh speed rotational operation and fretting wear will be reduced.Consequently, densification of the hard disk drive device can beachieved.

SUMMARY OF THE INVENTION

[0021] These and other objects are achieved by a spindle motor includinga bearing means of the present invention.

[0022] In a spindle motor in accordance to the first aspect of thepresent invention, the spindle motor comprising a spindle shaft, arotor, and a bearing means interposed between the spindle shaft and therotor, the bearing means includes an inner and outer rings and rollingelements interposed therebetween, characterized in that the inner andouter rings of the bearing means are made of steel, and the rollingelements are made of ceramic.

[0023] The spindle motor in accordance with the second aspect of thepresent invention has a structure that the spindle shaft is a straightshaft, and the bearing means is the ball bearing.

[0024] The spindle motor in accordance with the third aspect of thepresent invention has a structure that the spindle shaft of is a steppedshaft including a larger diameter portion and a reduced diameterportion, the outer ring is a cylindrical one surrounding the steppedshaft, the inner ring has an inner raceway on its outer peripheralsurface is fit around the reduced diameter portion of the spindle shaftand secured thereto, the bearing means further includes rolling elementsinterposed between the inner raceway and an outer raceway formed on theinner periphery of the outer ring, and rolling elements interposedbetween an inner raceway formed directly on the larger diameter portionof the spindle shaft and another outer raceway formed on the innerperiphery of the outer ring.

[0025] The spindle motors of the above mentioned structure aresubstantially the same as those of the spindle motor including a bearingmeans of the prior art in that the rotor is rotated by being energized.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Further, feature of the present invention will become apparent tothose skilled in the art to which the present invention relates fromreading the following specification with reference to the accompanyingdrawings, in which:

[0027]FIG. 1 is a longitudinal cross sectional view showing the firstembodiment of the spindle motor in accordance with the presentinvention;

[0028]FIG. 2 is a longitudinal cross sectional view showing the bearingmeans employed in the spindle motor of the first embodiment;

[0029]FIG. 3 is a longitudinal cross sectional view showing the secondembodiment of the spindle motor in accordance with the presentinvention; and

[0030]FIG. 4 is a table showing the results obtained from themeasurement on the variety of characteristics of the spindle motor inaccordance with the present invention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

[0031] The preferred embodiments of a spindle motor in accordance withthe present invention will now be described.

THE FIRST EMBODIMENT

[0032] A spindle motor 1 of the first embodiment as shown in FIG. 1includes a spindle shaft 2 secured to a base plate 9 to extendingvertically therefrom, a rotor 3 of a sleeve like configuration, and apair of upper and lower bearing means 4 interposed between the outerperipheral surface of the spindle shaft 2 and the inner peripheralsurface of a vertically extending central through bore 3 b of the rotor3. Thus the rotor 3 can rotate in high speed around the spindle shaft 2.

[0033] Each of the bearing means 4 is a conventional ball bearing, andthe spindle shaft 2 is a straight shaft.

[0034] A sealing member 5 of rubber for preventing dirt or dust fromimmigration into the bearing means 4 is provided above the upper bearingmeans 4 between the spindle shaft 2 and the rotor 3.

[0035] An annular recess 3 a is provided on the lower surface of therotor 3 to accommodate a stator 7 around which a coil 6 is wound. Thestator 7 is supported by a protrusion 9 a formed on the upper surface ofthe base plate.

[0036] A ring shaped magnet 8 is secured on the inner periphery of therecess 3 a so as not to contact with the stator 7.

[0037] Upon supplying the alternative current from the control circuit(not shown) to the coil 6 to generate a magnetic field from the pole ofthe stator 7, the rotor 3 including the magnet 8 is rotated. The loadfrom the rotation is to be supported by the spindle shaft 2 through thebearing means 4.

[0038] Each bearing means 4 includes an inner ring 10 having an innerraceway 10 a on its outer peripheral surface, an outer ring 11 having anouter raceway 11 a on its inner peripheral surface, and a plurality ofrolling elements which are shown as balls 13 interposed between theinner raceway 10 a and the outer raceway 11 a and supported in theirposition by a retainer 12.

[0039] A pair of sealing plates 14 are provided on both side of theballs 13 to prevent the grease injected between the inner raceway 10 aand the outer raceway 11 a from splashing out.

[0040] The inner and outer rings 10, 11 of the bearing means 4 are madeof high carbon chromium steel (SUJ 2 or the one similar to SUJ 2), andthe hardness of each surface of the inner raceway 10 a and the outerraceway 11 a is made to HRC 62. The rolling elements shown as balls 13are made of ceramic material such as silicon nitride, and the hardnessof the surface thereof is made to HRC 75.

[0041] The structure of the retainer 12 and the sealing plates 14 aresubstantially the same as those employed in the prior art.

[0042] A comparison is made between the first embodiment in which ballsof ceramic material are used and the prior art in which balls of highcarbon chromium steel are used.

[0043] The comparison is made under the condition that the grease usedin the bearing means of the spindle motor of the present invention andthat used in the bearing means of the prior art are equal in theirquality and quantity. The number of revolutions is set to 5400 rpm.

[0044] The measurement of the variety of characteristics of the spindlemotor is made on the noise, the non-repetitive run out (NRRO) in theaxial direction, the value of vibration level (G-value), and the runcurrent of the motor. The results obtained therefrom are listed on FIG.4.

[0045] As shown in FIG. 4, the characteristics of the spindle motor 1 ofthe present invention is that the noise is lower, the non-repetitive runout (NRRO) in axial direction, the G-value, the run current of the motorthan those of the spindle motor in which incorporated the bearing meansof the prior art the balls of which are of high carbon chromium steel.

[0046] As can be seen from above, the hard disk drive device includingthe spindle motor of the present invention shows increaseddensification, reduced electric energy consumption, and improvedacoustic characteristics.

[0047] The centrifugal force F acting upon the rolling element when thespindle motor is operated in high rotational speed can be expressed bythe following equation:

F=M(dp/2) (ωm)²

[0048] where the mass of the rolling element is M (kg), the pitchdiameter of the rolling element is dp (m), the angular velocity (rad/s)of the rolling element around the axis of the spindle shaft is ωm.

[0049] The density of the high carbon chromium steel used in making theballs of the bearing means of the prior art incorporated into thespindle motor is about 7.8, whereas the density of the silicon nitrideused in making the balls of the bearing means of the present inventionincorporated into the spindle motor is about 3.2, i.e. less than half ofthe density of the high carbon chromium steel.

[0050] In this connection, the mass of the balls can be reduced todecrease the centrifugal force to be applied on the balls upon operatedin high rotational speed. Thus the load to be applied due to thecentrifugal force of the balls on the outer ring can also be reduced.

[0051] It is therefore possible to avoid the damage on the raceway ofthe outer ring. It is further possible to reduce the power consumptionof the spindle motor since the inertia force is reduced by reducing themass of balls.

[0052] The balls of ceramic material are improved in their wearresistance and heat resistance relative to those of prior art made ofhigh carbon chromium steel, so that the seizure does not happen even onthe initial lubrication. Further, the seizure does not happen even ifthe oil film formed between the contact area between the surface of theraceway and the rolling element is broken. Such breakage of the oil filmis often caused by the reduction of the viscosity of the grease due tothe heating of the ball bearing upon rotating in high speed.

[0053] Although in the bearing means 4 as shown in FIG. 2, sealingplates 14 are attached on both end of each bearing means, in thestructure as shown in FIG. 1 in which the bearing means are incorporatedinto the spindle motor, it might of course be possible to provide thesealing plate 14 only on the side of each bearing means exposed to theoutside thereof.

[0054] In such structure in which the sealing plate 14 is attached onlyon one side of the bearing, the inner sealing plates are unnecessary,and the number of components can be reduced, and the cost formanufacturing can also be reduced thereby.

THE SECOND EMBODIMENT

[0055] The spindle motor of the second embodiment has a spindle shaft 16of stepped structure in which a reduced diameter portion 16 a and alarger diameter portion 16 b are provided.

[0056] A bearing means 21 including the stepped shaft further comprisesan outer ring 18 of cylindrical configuration disposed around thespindle shaft 16, and an inner ring 17 having an inner raceway 17 aformed on its outer peripheral surface. The inner ring 17 is fit aroundthe reduced diameter portion of the stepped shaft and secured thereto.

[0057] A pair of double row outer raceways 18 a, 18 b are formed on aninner peripheral surface of the outer ring 18. Balls 20 are interposedbetween the one outer raceway 18 a and the inner raceway 17 a, andretained in their position by a retainer 19.

[0058] Further, balls 23 are interposed between an inner raceway 16 cformed around an outer peripheral surface of the larger diameter portion16 b of the spindle shaft and the other outer raceway 18 b, and retainedin their position by a retainer 22.

[0059] The outer periphery of the cylindrical outer ring 18 is fitintegrally within the central through bore 3 b of the rotor 3.

[0060] Other structural elements of the bearing means of the secondembodiment which are essentially the same as those of the firstembodiment are designated by the same reference numerals as thoseemployed in the first embodiment, and the descriptions thereon areomitted.

[0061] In this embodiment, the spindle shaft 16, the inner and outerrings 17 and 18 of the bearing means are made of high carbon chromiumsteel (SUJ 2 or the one similar to SUJ 2), and the rolling elements orballs 20, 23 are made of ceramic material such as silicon nitride, andthe hardness of the surface thereof is made to HRC 75.

[0062] Regarding the spindle motor 15 of such structure, a comparisonand measurement is also made relative to the prior art in the samemanner as mentioned above. The measured values of the noise, thenon-repetitive run out in the axial direction, the G-value, and the runcurrent of the motor are confirmed to be reduced as the results asobtained on FIG. 4.

[0063] Although the bearing means 21 incorporated into the spindle motoras shown in FIG. 3 has the structure in which no sealing plates areprovided, the structure in which the sealing plates are attached toprevent the injected grease from splashing out can also be adopted.

[0064] Although the silicon nitride material is used to form the ballsin the above mentioned embodiments, the balls can also be made of thefine ceramics such as alumina, zirconia, silicon carbide, and so on.

[0065] Further, only balls are made of ceramic material, the inner andouter rings can also be made of ceramic material.

The Effects of the Invention

[0066] The present invention is a spindle motor of the above describedstructure into which a bearing apparatus including inner and outer ringsof steel and the rolling elements of ceramic material is incorporated.In this connection, the raceway surfaces of the inner and outer rings aswell as the surfaces of the rolling elements are hard to be damaged, thecentrifugal forces to be applied on the rolling elements do not increaseeven if the spindle motor is rotated in high speed, so that the load tobe applied on the outer ring can also be limited, the acousticcharacteristics (the level of the noise generated) and/or the vibrationcharacteristics are improved, and the lifetime of bearing means can beelongated.

[0067] While particular embodiments of the present invention have beenillustrated and described, it should be obvious to those skilled in theart that various changes and modifications can be made without departingfrom the spirit and scope of the invention.

What is claimed is:
 1. A spindle motor comprising a spindle shaft, arotor, and a bearing means interposed between the spindle shaft and therotor, the bearing means including inner and outer rings and rollingelements interposed therebetween, wherein the inner and outer rings ofthe bearing means are made of steel, and the rolling elements are madeof ceramic.
 2. The spindle motor as claimed in claim 1 wherein thespindle shaft is a straight shaft, and the bearing means is the ballbearing.
 3. The spindle motor as claimed in claim 1 wherein the spindleshaft is a stepped shaft including a larger diameter portion and areduced diameter portion, the outer ring is a cylindrical onesurrounding the stepped shaft, the inner ring has an inner raceway onits outer peripheral surface and is fit around the reduced diameterportion of the spindle shaft and secured thereto, the bearing meansfurther includes rolling elements interposed between the inner racewayand an outer raceway formed on the inner periphery of the outer ring,and rolling elements interposed between an inner raceway formed directlyon the larger diameter portion of the spindle shaft and another outerraceway formed on the inner periphery of the outer ring.